Resistance of early stationary phase E. coli to membrane permeabilization by the antimicrobial peptide Cecropin A

Antimicrobial peptides (AMPs) cause bacterial membrane permeabilization and ultimately cell death at low μM concentrations. The membrane permeabilization action of a moth derived AMP Cecropin A on E. coli cells in exponential growth (mid-log phase) is well studied. At 1× MIC concentration, Cecropin A penetrates the lipopolysaccharide (LPS) barrier and causes outer membrane (OM) and cytoplasmic membrane (CM) permeabilization. For non-septating cells, permeabilization of both membranes begins at one pole. For septating cells, OM permeabilization begins at the septal region and CM permeabilization begins at one pole. However, in nature bacteria are frequently found in nutrient-starved conditions. Here we extend our single-cell microscopy assays to the attack of Cecropin A on E. coli cells in early stationary phase. Stationary phase E. coli is much more resistant to membrane permeabilization by Cecropin A than mid-log phase E. coli. A tenfold higher concentration of Cecropin A is required to observe CM permeabilization in the majority of stationary phase cells, and even then permeabilization proceeds more slowly. In addition, the spatial pattern of initial CM permeabilization changes from localized at one pole to global. Studies of lipid mutant strains suggest that a sufficient localized concentration of the anionic phospholipid phosphatidylglycerol (PG) guides the position of initial attack of the cationic AMP Cecropin A on the CM.